Infections caused by the obligate intracellular bacterium Chlamydia trachomatis are among the most prevalent causes of genital and ocular diseases worldwide. Chlamydial infections can progress to chronic inflammatory sequelae that lead to infertility or blindness. The immune mechanisms that augment chronic inflammation are ill-defined, but repeated exposure to chlamydial antigens is thought to contribute to the disease process. Although the importance of repeated infection in the development of the chronic inflammatory sequelae of chlamydial disease has been established, persistent infection might also serve as a source of antigen for the chronic inflammatory responses. The primary objective of our studies this past year have been to develop and define an in vitro cell culture model of persistent chlamydial infection. Understanding persistent chlamydial in vitro may help provide us with a better understanding of the pathogenesis of chlamydial disease in vivo. An in vitro cell culture system was used to study the effect interferon- gamma (IFN-gamma) on Chlamydia trachomatis growth and differentiation. The effect of IFN-gamma was dose dependent. High concentrations (2 ng/ml) of IFN-gamma completely inhibited chlamydial growth and differentiation, whereas persistent infection was established when chlamydia infected cells were cultured with 0.2 to 0.5 ng/ml IFN-gamma. Persistent infection was characterized by the development of noninfectious atypical chlamydial forms from which infectious progeny could be recovered only when IFN-gamma was removed from the culture system. Analysis of persistently infected cells by immunofluorescent microscopy and immunoblotting with specific antibodies revealed that the atypical chlamydial forms had near normal levels of the 60-kDa heat shock protein, an immunopathologic antigen, and a paucity of the major outer membrane protein, a protective antigen. If IFN-gamma causes similar events to occur in vivo, then persistently infected cells could augment the pathogenesis of the chronic inflammatory sequelae that follow chlamydial infection by serving as depots of antigen capable of stimulating a sustained inflammatory response.